Well, R0 is by definition different for different populations - density, demographics, etc. R0 and the herd immunity threshold have a clear mathematical relationship; it's just a restating of the definition. So if you argue the threshold is lower for a subpopulation, you're also arguing that R0 is lower for that same subpopulation.

It also depends on the interconnectedness of the population. Person A may not be very connected, only being particularly likely to infect their own household, and maybe one or two other people outside. Person B might be highly connected and in a position to infect dozens of people. If you had a population of 80% "Person A" and 20% "Person B", this might average out to an R0 of around 6 -- but the actual reproductive rate would drop rapidly as "Person B"s gain immunity.

That makes intuitive sense to me, that over the lifetime of an unrestrained virus propagation, the R0 value would start out higher and slowly decrease until the herd immunity threshold is reached. Not sure if that is commonly observed in the epi community but it wouldn't surprise me.